The present invention relates to a system and method for controlling rotor blade over-tip leakage of a working fluid in a rotating machine.
The working fluid of a gas turbine engine is operated on by circumferential rows of turbine and compressor blades which rotate within static casings. To achieve high efficiencies it is important that over-tip leakage by the working fluid (i.e. leakage between a blade and a casing) is minimised by letting the rotating blades run as close as possible to the casings.
Blades can be shroudless or can have shrouds with radially outer sealing fins, but both have the same problem of over-tip leakage.
Considering a row of conventional shrouded turbine blades with seal fins, the part of the corresponding casing closest to the fin tips is typically formed by a circumferential row of seal segments which are hooked into the casing. Each seal segment has a layer of abradable material facing inboard to the fins. The fin tips are normally designed to rub into the abradable material to form a wear track that provides a seal against over-tip leakage. To accommodate differential thermal expansion effects and inevitable variation in radial position of the individual segments, the abradable material has a thickness well larger than the predicted maximum incursion. The fin tips are often coated with a resistant material so that when they come in contact with the abradable material, the abradable material is rubbed away preferentially to the tips.
However, due to build and assembly tolerances, there can be variation in the radial positioning of the seal segments and in the radial positioning of the blade tips. Also, each segment has a tendency to flatten during engine operation due to the temperature difference across the segment, resulting in a slightly higher hot running radius at the ends of each segment than in the centre. In addition, the casing into which the segments hook may not be perfectly circular.
For an optimal (i.e. minimised) clearance, circumferentially continuous rubs are required as this ensures that the radial differences of the segments are eliminated. However, the deepest rub is created by the longest blade as it sweeps around, resulting in a performance deficit proportional to the averaged gap due to the variation in fin tip length.
By using a suitable thickness of abradable material and choosing a tight build clearance, some of the difficulties associated with variation in the radial positioning of the seal segments, segment flattening, and casing non-circularity can be addressed (although it is not unusual to see engines without all-round rubs or rubs only at the centre of the seal segments). However, conventional sealing systems struggle to address also variation in blade tip radial positioning.
In general terms, the present invention provides a system and method for controlling rotor blade over-tip leakage of a working fluid in a rotating machine in which not only is an abradable layer abraded by the blade tips to form a wear track for the tips having a uniform radius, but also means are provided to wear down the blade tips to provide a uniform blade tip radius.
By forming the wear track with a uniform radius to match the uniform blade tip radius, an optimal, or close to optimal, clearance can be produced at each tip to reduce over-tip leakage and improve operating efficiencies.